The method for serial communication has been known as one of the methods for transmitting data serially, and many variations of the method have been devised. Serial data communication is characterized by that it requires a comparatively few communication lines, and that it allows comparatively easy data control. Because of these features, the method for serial data communication has been widely adopted by the communication systems of various fields.
However, serial data communication requires a comparatively long time if data to be transmitted are large in volume. Further, if a series of data have been dispatched, and a change must be added to the data later, usually all the data including the change must be transmitted anew. Thus, even if the change only involves a single data of the previous data, all the previous data and clock signals corresponding with those data must be transmitted anew. Because of these inconveniences, the conventional serial data communication requires a comparatively long time particularly when a data change must be added later.
Here description will be given below about the problems encountered with the conventional serial data communication, taking as an example an image forming system based on thermal ink jet printing. According to the thermal ink jet-based image forming system, a driving pulse is applied to an electro-thermal transducer element attached to an ink ejection nozzle, thereby causing a bubble to develop in ink within the ink passage leading into the nozzle; the bubble gives a sufficiently high pressure upon the ink droplet remaining close to the nozzle, to force it out as an ink jet; and the ink jet is allowed to hit against a sheet of recording paper, to print a dot there.
Such an image forming system adopts serial signal transmission in order to transmit printing data from an engine portion to a recording head portion. The engine portion or the transmitter delivers, for example, data necessary for printing a line to a memory means on the side of the receiver. For this data transmission to be properly achieved through serial data communication, clock signals are used for informing which data should be assigned to which locations of the memory means. If the memory means has, for example, eight addresses, each address having a 1 byte width, the number of cells to accept data will count 8×8 or 64 in total. Thus, if it is required for the system to transmit data necessary for one line printing, the system will transmit 64 clock signals through a clock line, and data correspondent with those clock signals through a data line, so that data necessary for printing a line can be properly stored in the memory means of the recording head portion. The image forming system taken as an example here for illustration requires at least 64 clock signals for the transmission of data necessary for printing a line. If the image forming system must print a full color image, it will incorporate four printer heads, and thus data necessary for printing a line of a color image will become large in volume, and the number of clock signals will increase in association, which will unduly lengthen the time necessary for printing.
The recording method based on ink jet printing poses another problem: the amount of the ink droplet will vary depending on the temperature of the ink jet element from which the droplet is spat out. In order to meet this situation, a temperature sensor is attached to each nozzle to detect the temperature of the nozzle, and a feedback control is introduced for altering the width of a driving pulse applied to the ink jet element according to the temperature of the element. Accordingly, the system may meet a situation where, after data necessary for printing have been delivered by the engine portion to the printing portion, the system decides to change the width of a driving pulse to be applied to a certain ink jet element in response to a detection signal from the temperature sensor of that element, and must add the change to a certain part of the data previously delivered. To put it more specifically, the system may meet a situation where, when formation of an image is started, and, during the formation, the temperature of a certain ink jet element is elevated, the system must reduce the width of a driving pulse to be applied to the ink jet element in question and add the change to the data previously sent. If the conventional image forming system meets such a situation, the system containing a memory means on the receiver side having 64 cells as described above, the system will send anew 64 clock signals, and the data to be changed in synchrony with a clock signal corresponding to a cell at which data to be changed are stored. As discussed above, according to the conventional serial data communication, even if a change must be added only to a single data out of 64 data, 64 clock signals must be sent anew together with the change, just as if all the data had to be changed. Namely, according to the conventional serial data communication, changing of the previously sent data requires an unduly long time.
This invention was proposed with the above situation as a background, and aims at providing a method for synchronous serial communication capable of increasing the transfer speed of serial data, and a system for synchronous serial communication based on the method.